Sea spider
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Sea spiders, also called Pantopoda or pycnogonids, are marine arthropods of class Pycnogonida. They are cosmopolitan, found especially in the Mediterranean and Caribbean Seas and the Arctic and Antarctic Oceans. There are approximately 1,000 known species, ranging in size from 1-10 mm to over 90 cm in deepwater species: most are toward the smaller end of this range.
The correct taxonomy within the group is uncertain, and it appears that no agreed list of orders exists. Accordingly, families are listed in the table at the right. Furthermore, the evolutionary lineage of the sea spider group as a whole is disputed: their fossil record is sketchy at best, with the three earliest known genera originating in the Devonian. Some believe (or once believed) sea spiders to be among the chelicerates, together with horseshoe crabs, true spiders, mites, ticks and scorpions. While sharing many morphological features with the other chelicerates, major differences (such as their unique proboscis) have caused some taxonomists to remove sea spiders from this grouping.
Their reduced body size is in contrast to large, long legs, which may be present in four or more pairs. Because of their small size and slender body and legs, no respiratory system is needed for gas diffusion. A proboscis allows them to suck nutrients from soft bodied invertebrates, and their digestive tract has branches extending into their legs, known as diverticulae.
Their habitats range from the shallow intertidal zones to deepwater benthic environments, some 7,000 metres down. Sea spiders do not swim but rather walk along the bottom with their stilt-like legs. Most are carnivorous and feed on cnidarians, hydroids, sponges, soft corals, polychaetes and bryozoans.
The order Pycnogonida consists of approximately 1000 species, which are normally split into eighty-six genera. These small animal, commonly know as “sea spiders,” live in many different parts of the world, from Australia, New Zealand, and the Pacific coast of the United States to the Mediterranean and the Caribbean to the north and south poles. They are found in waters as deep as 7000 meters, but more commonly in shallower waters and in both marine and estuarial habitats. Although they are relatively common, Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.
Usually measuring between one and ten millimeters, they crawl slowly along (although some do swim), feeding on hydroids, soft corals, anemones, bryozoans, and sponges. Sea spiders are generally predators or scavengers. They will often insert their proboscis, a long appendage used for digestion and sucking food into its gut, into a sea anemone and suck out nourishment. Amazingly, the sea anemone, relatively large in comparison to its predator, almost always survives this ordeal. Studies that adult taste preferences depend on what the animals were fed as young.
Pycnogonids are so small that each of their tiny muscles consists of only one single cell, of course “surrounded by connective tissue”. In terms of physical makeup, the proboscis, which has fairly limited dorso-ventral and lateral movement, and three to four appendages including the ovigers, which are used in caring for young and cleaning as well as courtship, are present in the anterior region. The last segment includes the anus and tubercule, which projects dorsally. In total, Pycnogonids have four to six legs for walking as well as other appendages which often resemble legs. A cephalothorax and much smaller abdomen make up the extremely reduced body of the Pycnogonid, which has four dorsally located simple eyes on its non-calcareous exoskeleton. The organs of this fascinating chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone.
The setup of the sea spider actually creates a perfect surface-area to volume ratio for any respiration to occur through direct diffusion. The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a molt. A Pycnogonids small heart, which is long and thin like everything else in each them5, beats “very vigorously” at ninety to 180 beats per minute, creating “substantial” blood pressure. These creatures possess an open circulatory system as well as a nervous system consisting of a brain, which is connected to two ventral nerve cords, which in turn connect to specific nerves.
All Pycnogonid species have two separate and different genders except for one species which is hermaphroditic. Females possess a pair of ovaries, while males possess a pair of testes located dorsally in relation to the digestive tract. Scientist are aware that reproduction involves external fertilization after “a brief courtship, but very little is know about the secret lives of most Pycnogonids”. Interestingly, only males care for laid eggs and young, a rather rare phenomenon in the animal world.
In regards to the evolution of Pycnogonids, the miniscule fossil record is quite lacking, although we two things are fairly clear:
1. Pycnogonids possessed a coelom at one point, but it was eventually lost through evolution, and
2. Pycnogonids are relatively old, with the earliest know genera placed in the Devonian period, between 416 and 355 million years ago.
This writer has decided to conclude with a “fun fact”: David Staples, one of the
only Pycnogonid specialists in the world, notes, “An early researcher in the [study of Pycnogonids] was T.T. Flynn, father of actor Errol Flynn”.
External links
- Introduction to the Pycnogonida (http://www.ucmp.berkeley.edu/arthropoda/pycnogonida.html)
- Sea Spiders (Pycnogonida or Pantopoda) Web Page (http://www.invertebrate.ws/seaspider/)
- Taxonomic hierarchy for the class Pycnogonida (http://erms.biol.soton.ac.uk/cgi-bin/hierarchy.pl?rank=class&taxon=Pycnogonida)
- Images of Pycnogonida (http://www.bioimages.org.uk/HTML/T118895.HTM)
- Images of Pycnogonida, and Pycnogonids in literature (http://sprott.physics.wisc.edu/pickover/pycno.htm)